As voltage is applied to or removed from electrorheological fluids, they stiffen or soften proportionally. Their unique property has been known for at least fifty years and has been used in conjunction with systems for damping the motion of a vibrating body. Systems include those, for example, that use sensors to sense tension or motion of a body in communication with electrorheological fluid. Various schemes have been used to control voltage so that it is applied or removed from the electrorheological fluid according to sensor information, thus causing a change in the viscosity of the fluid, and therefore altering its stiffness.
Well known examples in machinery dynamics using electrorheological fluids are noise-isolating suspensions for motors, transmissions, drives and shock absorbers for vehicles; vibration isolators for rotor bearings; vibration isolators in antennas, etc. Among electromechanical examples are vibration isolators for pickups in magnetic storage devices of computers, and impulse and vibration isolators for sensitive measuring devices. In structural engineering are oscillation isolators for bridges, and base isolation device for buildings subject to seismic disturbances.
These systems for vibration suppression are primarily based on "passive" measures such as tuning of parameters. That is, system parameters such as spring and damper constants, masses, etc. are chosen in some optimal fashion, e.g. so as to minimize the amplitude of oscillation of a critical system component. An improvement over the "passive" systems are systems employing "active" measures such as hydraulic or electromagnetic actuators. These and similar techniques suffer from a number of disadvantages. Specifically, they are costly, slow and complicated.